Which of the following is true regarding membrane proteins?
a. all peripheral membrane proteins contact the lipid bilayer by covalent attachment to the hydrophilic head groups on membrane lipids
b. beta strands can assemble into a membrane-spanning barrel structure.
c. lipid-anchored membrane proteins always face the cell exterior.
d. a protein with six hydrophobic helices is most likely a lipid-anchored membrane protein .

Option C. Lipid-anchored membrane proteins always face the cell exterior.

Explanation:

Lipid-anchored proteins are proteins located on the surface of the cell membrane which are attached to lipid and embedded within the cell membrane. The lipid-anchored proteins assume a place in the bilayer structure of the membrane.

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answer; both surfaces of the membrane are nonpermeable